Patterns and processes of tree mortality in montane forests of northern Arizona

I investigated the patterns and processes of tree mortality in mixed-conifer and ponderosa pine forests of northern Arizona. Specifically, I investigated the abiotic and biotic factors that contribute to tree mortality of white fir (Abies concolor), trembling aspen (Populus tremuloides ), limber pine (Pinus flexilis), and Douglas-fir (Pseudotsuga menziesii) at the tree, stand, and site scales in mixed-conifer forests. Additionally, I studied the patterns of allocation between growth and defense in the xylem of live and dead ponderosa pines of the region.;At the stand scale, I found that a recent prolonged drought (1996-2003) was associated with 60-90% of recent tree mortality in mixed-conifer forests of northern Arizona. Aspen and white fir experienced the most tree mortality (> 30%) over all sites. Tree mortality during this time period was delayed by 1 to 4 y relative to climatic variables. In all, tree mortality in mixed-conifer forests was related to a complex set of factors, which varied markedly by site and species. Topography, stand characteristics, and biotic factors were variably important at different sites and species at the plot-scale.;At the tree scale, growth (ring width) patterns differed substantially between live and dead trees for all tree species of mixed-conifer forests over all sites. In most cases, dead trees had consistently less growth over the past 50 years. Mortality models for all species indicated that both mean growth and growth trends (rate of growth over time) were strongly related to tree death. Dead trees of all species studied often had >50 % less growth compared to live trees. The number of abrupt declines (greater than 50 % decrease in growth between years) improved mortality models of some tree species but was less informative than mean growth and trend characteristics. These results indicate that tree death is related to growth patterns over short and long time scales, which places the mixed-conifer forest tree species I investigated along an intermediate position between the sudden death hypothesis and the gradual decline hypothesis.;Comparisons of growth (ring width) and defense (xylem resin duct characteristics) of live and dead ponderosa pines indicated that defense allocation was relatively more important to tree survival than tree growth. While growth was slightly greater in living trees, these differences were not statistically significant. However, resin ducts in live trees were larger (>10 %), denser (>25 %), and composed more area (>50 %) of xylem than dead trees, which resulted in a greater likelihood of survival. While we found that growth and defense were strongly correlated, defense was more informative in modeling tree survival.;Tree mortality is a complex process involving numerous factors at the tree, stand, and site scales. Models of tree mortality for a given species will need to incorporate factors at all of these scales to predict future mortality events. Climatic projections suggest that continued warming and increased droughts would contribute to further tree mortality, similar to or more severe than this recent mortality event in the region (i.e. 1996-2003).;Information on the patterns and processes of tree mortality at multiple scales will be of use to managers interested in reducing the impacts of drought-triggered tree mortality. In general, our research indicates that managers should focus on prioritizing treatments on particular sites and species to mitigate future drought related tree mortality. Specifically, we suggest that thinning areas with high basal area of white fir or Douglas-fir will reduce drought related tree mortality. In stands containing aspen, reducing the amount of conifer basal area will reduce tree mortality. Furthermore, managers should conduct treatments that promote greater individual tree growth or, in some tree species (e.g. ponderosa pine) greater allocation to defense (e.g. resin ducts). (Abstract shortened by UMI.).